For the baseline-category logit model without constraints on parameters,

????ij = exp(xi????j

)

∑c h=1 exp(xi????h)

, show that dividing numerator and denominator by exp(xi????c) yields new parameters ????∗

j = ????j − ????c that satisfy ????∗

c = 0. Thus, without loss of generality, we can take ????c = 0.